In an ancient, orbital dance that has been ongoing for several billion years, the Galilean satellites Europa and Callisto are caught, under the watchful gaze of Jupiter, nearly perfectly aligned with each otherand the planet's center in this true color frame made of narrow angle images taken on December 7, 2000 at 16:11 UTC (spacecraft event time). The distances here are deceiving. Europa (seen against Jupiter) is 600,000 km above the planet's cloud tops; Callisto (at lower left) is nearly three times that distance at 1.8 million km.

Europa is a bit smaller than our Moon and is one of the brightest objects in the solar system. Callisto is 50% bigger -- roughly the size of Saturn's largest satellite Titan -- and three times darker than Europa: its brightness had to be enhanced relative to that of Europa and Jupiter in order to see it in this image.

These objects, which have had very different geologic histories, share some surprising similarities. While they both have surfaces rich in water ice, Callisto has apparently not undergone major internal compositional stratification, whereas Europa's interior has differentiated into a rocky core and an outer layer of nearly pure water ice. Callisto's ancient surface is completely covered by large impact craters: the brightest features seen on Callisto in this image were discovered by the Voyager spacecraft in 1979 to be bright rayed craters, like those on our Moon. In contrast, Europa's young surface is covered by a wild tapestry of ridges and chaotic terrain and only a handful of large craters.

Surprisingly, data from the magnetometer carried by the Galileo spacecraft, which has been in orbit around Jupiter since 1995, indicate the presence of conducting fluid, most likely salty water, inside both worlds. (A similar Galileo discovery has been made of Ganymede, to be featured tomorrow.)

Does the surface of Saturn's Titan resemble that of Callisto or Europa, or will it be entirely different? We'll find out in 2004 when Cassini finally reaches its destination.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.